The present invention relates to determination of a property of an optical device under test, e.g. the determination of the elements of the Jones Matrix of an optical device. The Jones matrix contains information about the optical properties of the device under test, which can be a fiber or an optical component. Knowledge of the Jones matrix of an optical component enables determination of many important optical properties of the component, such as insertion loss, reflectivity or transmissivity, polarization dependent loss (PDL), and polarization mode dispersion (PMD).
PMD is a fundamental property of single mode optical fiber and components in which signal energy at a given wavelength is resolved into two orthogonal polarization modes of slightly different propagation velocity. The resulting difference in propagation time between polarization modes is called the differential group delay, or DGD. The term PMD is used to denote the physical phenomenon in general and the mean, or expected, value of DGD in particular. The attributes that define PMD are DGD, and the principal states of polarization, or PSP. Both are generally functions of wavelength in single mode fiber systems. Long fibers typically exhibit random polarization coupling, and consequently, PMD scales with the square root of fiber length for fibers longer than several kilometers. State of the art fiber may be limited to a few tenths of picoseconds of DGD per root kilometer. Additionally, state of the art components for such fiber communication systems may exhibit only tenths of picoseconds of DGD.
PMD causes a number of serious capacity impairments, including pulse broadening. In this respect its effects resemble those of a chromatic dispersion, but there is an important difference. Chromatic dispersion results from a variation in propagation delay with wavelength caused by the interplay of fiber material and dimensions and is a relatively stable phenomenon. The total chromatic dispersion of a communications system can be calculated from the sum of its parts, and the location and value of dispersion compensators can be planned in advance. In contrast, the PMD of single mode optical fiber at any given signal wavelength is not stable, forcing communications system designers to make statistical predictions of the effects of PMD and making passive compensation impossible. Moreover, PMD becomes a limiting factor after chromatic dispersion has been sufficiently reduced.
This is because the channel PMD, i.e. the mean value of DGD for the fiber over wavelength and time, also called the expected value, can often be in excess of 20 ps. This value is within the bit resolution of a 40 Gbit/s communication system, and as a result, the communication system is adversely affected by the PMD. Additionally, in state of the art communication systems components are often introduced in cascades, e.g. by introducing a cascade of a great number of Bragg gratings in the fibers. Although the single component of such a cascade may exhibit only tenths of picoseconds of DGD the total cascade may exhibit DGDs which reach the resolution of the transmission rate. Therefore, it becomes more and more necessary to be able to gain exact information about the PMD of each single component.
The aforementioned problem has inspired the development of many measurement methods to measure PMD. In the following a few methods of the known methods shall be discussed.
In the fixed analyzer PMD measurement method PMD is determined statistically from the number of peaks and valleys in the optical power transmission through a polarizer as wavelength is scanned. A polarizer placed directly before a detector is referred to as an analyzer, hence the name of the method. The fixed analyzer response may be Fourier transformed to yield a spectrum that gives insight into the degree of mode coupling and allows calculation of PMD from a Gaussian fit or from the second-moment algorithm. The problem with the fixed analyzer method is that it is not possible to measure the PMD of components which exhibit band widths which are smaller than the variation in the optical power transmission over wavelength.
Another method is the interferometric method, which determines PMD from the electric field autocorrelation function using a broad band source. The value of PMD is computed with an algorithm based on the second moment. The problem of this method is that it only produces exact values of PMD when the PMD is caused by pure birefringence. However, this method is not able to produce useful PMD values when the PMD is wavelength dependent.
Another method is the so called Poincarxc3xa9 arc or SOP (state of polarization) method, which uses a polarimeter to capture the arc traced out on the poincarxc3xa9 sphere by the output polarization of the test device over a series of wavelength increments. However, if the polarized light is coupled accidentally into the main state of polarization of the test device no PMD can be measured. Another problem is that a high-resolution polarimeter is necessary which kind of polarimeters tend to be very expensive. Moreover, with this method no chromatic dispersion can be measured.
Another method is the so called Jones matrix eigenanalysis or JME method. This method determines DGD and PSP as functions of wavelength from measurements of the transmission matrix at a series of wavelengths. Again, this method uses an expensive polarimeter. This method gives no information about chromatic dispersion, neither.
Finally, there are methods known which measure the PMD more or less on a direct way. These methods, e.g. the modulation phase method and the pulse-delay methods determine PMD from measurements of the change in modulation phase and the change in pulse arrival time, respectively, between the principal states of polarization. The drawback of these methods is the pulse shape dependency of the results.
Therefore, it is an object of the invention to enable an improved determination of properties of an optical device.
The object is solved by the independent claims.
An advantage of the present invention is the possibility of deriving transmissive and reflective properties, e.g. the PMD of the device under test (DUT) just by determining the elements of the Jones matrix (which will be explained in more detail further below) of the DUT without need to make use of an expensive polarimeter, and the possibility of simultaneously measuring the chromatic dispersion of the DUT. So all the above-mentioned problems in the prior art can be avoided by the present invention. Moreover, it is possible to derive additional information from the derived Jones Matrix of the DUT. The present invention is capable of determining from the Jones matrix of a DUT as a function of wavelength during just one measurement. From the measured Jones matrix, it is possible to determine the insertion loss, reflectivity or transmissivity, polarization dependent loss (PDL), group delay, chromatic dispersion, differential group delay, principal states of polarization, and higher-order PMD parameters.
It is preferred to use a Jones Matrix. The information in the Jones matrix can be represented in several different ways, one alternative is called the Mueller matrix, which carries nearly the same information as the Jones matrix but represents it in a 4xc3x974 matrix with all real elements. However, absolute phase properties and consequently chromatic dispersion cannot be described by the Mueller matrix. There are other ways to achieve the same goal. One topic of the invention is a way to measure PMD, PDL, PSPs, DGD, group delay, chromatic dispersion, etc. with the interferometric method and apparatus described. However, it is possible to perform the inventive derivation with other tensors than with the Jones Matrix.
The term xe2x80x9ccoherentxe2x80x9d in this application means that the coherence length of the incoming light beam is bigger than the difference of lengths of the paths of the light beams to be superimposed.
In a preferred embodiment of the invention the apparatus contains a first Mach-Zehnder interferometer whereby a polarization setting tool is placed in the measurement arm so that the laser light couples into the DUT with a defined polarization. This direction of polarization is then defined as the x-axis of the coordinate system of the Jones matrix calculus. Accordingly, the first two elements of the Jones matrix can easily be derived. In a second run of the inventive method the other two elements of the Jones matrix are derived with the same interferometer by changing the direction of polarization of the light beam incident on the DUT. It is preferred for ease of evaluation the results to change the polarization to a polarization orthogonal with respect to the former polarization. In this respect, it is further preferred that the initial polarization is linear and the changed polarization is changed by 90xc2x0 with respect to the initial polarization.
In a further preferred embodiment the second run of the inventive method to determine the other two elements of the Jones matrix is not necessary. The Jones matrix is completely determined with just one run. For a complete acquisition of the Jones matrix it is necessary to test the DUT with two orthogonal input polarizations. Therefore, in this preferred embodiment a so-called xe2x80x9csingle scanxe2x80x9d measurement concept is proposed. In this concept a polarization delay step generates two, preferably orthogonal, polarized signals which are delayed with respect to each other. The polarization delay-step provides two polarization states which are fed simultaneously into the DUT and which are xe2x80x9ccodedxe2x80x9d by different propagation delays. Preferably, to delay the two signals with respect to each other the first light beam is split into a first undelayed light beam and a first delayed light beam in the polarization delay step. Subsequently both polarizations or parts of the first light beam are superimposed with the light of the local oscillator, i.e. the second light beam. Preferably, the two parts of the first light beam are orthogonal polarized with respect to each other so that they do not interfere when being superimposed. However, the two polarizations create interference patterns when being superimposed with the second light beam. Because of the delay introduced by the polarization delay step, the detected interference signals will have two distinct frequencies. These interference frequencies can be individually isolated and analyzed using electronic or digital filters. The analysis performed enables a determination of the elements of the Jones matrix of the optical device. There are many advantages linked to the aforementioned embodiment of the single-scan concept. First of all it is possible in the embodiments in which two scans are performed that the DUT changes its optical properties during the time between scans. Therefore, the stability requirements of the DUT are quite high. Moreover, performing two scans requires a lot of time so that with the single-scan concept the measurement speed can be increased dramatically. Furthermore, the measurement accuracy can be impaired by vibration noise, phase noise or a disturbance of the optical source between the measurements. This is also avoided by present embodiment. As a further advantage of the present embodiment the required precision with which each datum is assigned a frequency is less than the precision required in the other embodiments performing two scans since in this embodiment the two scans are performed simultaneously and therefore both relate to a certain frequency of the laser source.
It is clear for the skilled person that many other methods for splitting the first light beam, delaying one part with respect to the other part of the first light beam, and recombining them with different, preferably orthogonal, polarizations could be imagined.
In a respective polarization delay unit to perform the polarization delay step there are preferably used two polarization beam splitters, one to split the first light beam in the first delayed and in the first undelayed light beam, and the second to recombine the first delayed and the first undelayed light beam. The two polarization beam splitters each having two ports are connected with each other by polarization maintaining fiber. One of these two fibers is longer than the other to create the aforementioned delay of one part of the split first light beam. Alternatively, the recombining polarization beam splitter can be replaced by a polarization maintaining coupler.
In another preferred embodiment of either the double or single scan methods, there is a second Mach-Zehnder interferometer parallel to the first one. In this second interferometer the same coherent laser beam of the laser source is coupled in by a beam splitter before these two interferometers. With the help of the second interferometer, which is a reference interferometer without an optical device in its measurement arm, any errors in the detected powers of the resulting beams of the first interferometer caused by a nonlinearity in the scanning velocity when scanning the frequency of the laser can be eliminated.
Instead of using a second interferometer it is possible to use the second port of the second polarization beam splitter which recombines the two parts of the first light beam to couple out a signal to be used for the wavelength measurement. This is done by connecting this port directly with a polarizer which causes interference between the two parts of the first light beam, so that an interference pattern is provided to evaluate the scanning velocity when scanning the frequency of the laser source. Since the DUT is connected with the other port of the polarization delay unit, this port is not influenced by the DUT and can therefore be used as a reference. An advantage of this embodiment is that it does not need a second interferometer which reduces costs, necessary maintenance and the footprint of the inventive apparatus.
Other preferred embodiments are shown by the dependent claims.
It is clear that the invention can be partly embodied or supported by one or more suitable software programs, which can be stored on or otherwise provided by any kind of data carrier, and which might be executed in or by any suitable data processing unit. The software program is preferably used to evaluate the optical properties from the measured data.